The invention relates to a process for maintaining a given zinc content in nickel-containing zinc phosphate baths and to the application of this process to phosphatizing baths having a low zinc content.
During the practical operation of zinc phosphate baths for the chemical surface treatment of metals, bath components are consumed by formation of coatings, sludge precipitation and mechanical bath losses. These losses are compensated by addition of supplementary phosphate concentrate of appropriate composition to the phosphatizing bath. For ascertaining the consumption of chemicals and the supplementary quantity required, the current bath concentration is determined and compared with the prescribed concentration.
A customary method consists in expressing the bath concentration by the consumption of ml of N/10 NaOH for the titration of a 10-ml bath sample against phenolphthalein as indicator, ("total acid number"). A further parameter is the electric conductivity which, similar to the total acid number, also permits only an overall statement about the total concentration.
Since the introduction of the "low zinc" phosphatizing processes (German Offenlegungsschrift No. 2,232,067, German Offenlegungsschrift No. 3,101,866), the determination of the zinc content and its maintenance within narrow limits has gained special importance. For this purpose, automatic titration apparatus is employed to ascertain the cation content of the bath with the aid of complexometry, the end point being indicated colorimetrically, and the measured value thus obtained is used for adjusting the bath concentration. Since most low zinc phosphatizing baths contain, apart from zinc, also nickel, this analytical method supplies a measured value which is inaccurate due to the nickel concentration.
It is an object of the invention to provide a process for maintaining the zinc concentration in nickel-containing zinc phosphate baths which does not have the disadvantages of the known processes, works accurately and does not involve any significant expenditure for control equipment.